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How to input UEF or EF of a water heater.

asked 2018-07-05 15:13:06 -0500

Lingkai's avatar

updated 2018-07-05 16:33:23 -0500

I'm wondering how to interpret UEF (Uniform Energy factor) or EF (Energy factor) for water heater in OS. There are thermal efficiency and on cycle loss fraction for input. Which is the best representative of UEF or EF?

Thanks for any advice.

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@Lingkai@shorowit Wouldn't it be nice if EnergyPlus accept UEF directly? UEF is the new, federally mandated standard, so I added a New Feature Request to the EnergyPlus github.

mldichter's avatar mldichter  ( 2020-09-15 12:18:16 -0500 )edit

@mldichter There is not a direct mapping of EF/UEF to physical water heater properties, hence there are different models w/ assumptions that can be used. I assume it's the same reason that EnergyPlus doesn't have direct inputs for air conditioner SEER, furnace HSPF, clothes washer IMEF, etc. It's probably more appropriate as an OpenStudio measure.

shorowit's avatar shorowit  ( 2020-09-15 13:00:06 -0500 )edit

@shorowit You're right. We're trying to back out properties of real life water heaters from the UEF, but we're having to make a lot of assumptions. Too bad the new UEF rating system, although nice for consumers comparing water heaters, is not conducive to modeling.

mldichter's avatar mldichter  ( 2020-09-19 01:38:54 -0500 )edit

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answered 2018-07-05 16:03:00 -0500

updated 2021-09-02 11:04:31 -0500

To go from the EF rating value to storage water heater inputs (tank loss and thermal efficiency), you'll want to use one of the following two models that disaggregate these inputs:

  • Water Heater Analysis Model (WHAM), developed by Jim Lutz
  • NREL model by Burch/Erickson

They both give pretty similar results.

Since the UEF didn't exist back when those models were created, they both start with EF as the input. If you want to start with UEF, you could use the equations provided by RESNET to first convert UEF to EF:

Edit: There is now a paper documenting the procedure for using UEF.

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Thanks Scott! it absolutely answers my question.

Lingkai's avatar Lingkai  ( 2018-07-05 17:05:30 -0500 )edit

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Asked: 2018-07-05 15:13:06 -0500

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Last updated: Sep 02 '21